An electric power steering apparatus which provides a steering mechanism of a vehicle with a steering assist torque (an assist torque) by means of a rotational torque of a motor, applies a driving force of the motor as the assist torque to a steering shaft or a rack shaft by means of a transmission mechanism such as gears or a belt through a reduction mechanism. In order to accurately generate the steering assist torque, such a conventional electric power steering apparatus (EPS) performs a feedback control of a motor current. The feedback control adjusts a voltage supplied to the motor so that a difference between a steering assist command value (a current command value) and a detected motor current value becomes small, and the adjustment of the voltage applied to the motor is generally performed by an adjustment of duty command values of a PWM control.
A general configuration of a conventional electric power steering apparatus will be described with reference to FIG. 1. As shown in FIG. 1, a column shaft (a steering shaft, a handle shaft) 2 connected to a steering wheel (a steering handle) 1, is connected to steered wheels 8L and 8R through reduction gears 3, universal joints 4a and 4b, a rack and pinion mechanism 5, and tie rods 6a and 6b, further via hub units 7a and 7b. Further, the column shaft 2 is provided with a torque sensor 10 for detecting a steering torque of the steering wheel 1, and a motor 20 for assisting the steering force of the steering wheel 1 is connected to the column shaft 2 through the reduction gears 3. Electric power is supplied to a control unit (ECU) 100 for controlling the electric power steering apparatus from a battery 13, and an ignition key signal is inputted into the control unit 100 through an ignition key 11. The control unit 100 calculates a current command value of an assist (steering assist) command based on a steering torque Tr detected by the torque sensor 10 and a vehicle speed Vel detected by a vehicle speed sensor 12, and controls a current supplied to the motor 20 based on a voltage control value E obtained by performing compensation and so on with respect to the current command value. Moreover, it is also possible to receive the vehicle speed Vel from a controller area network (CAN) and so on.
The control unit 100 mainly comprises a CPU (or an MPU or an MCU), and general functions performed by programs within the CPU are shown in FIG. 2.
Functions and operations of the control unit 100 will be described with reference to FIG. 2. As shown in FIG. 2, the steering torque Tr detected by the torque sensor 10 and the vehicle speed Vel from the vehicle speed sensor 12 are inputted into a current command value calculating section 101, and a current command value Iref1 is calculated by means of an assist map and so on. The calculated current command value Iref1 is inputted into a maximum output limiting section 102 and an output is limited based on an overheat protection condition or the like in the maximum output limiting section 102. A current command value Iref2 that a maximum output is limited, is inputted into a subtracting section 103. Moreover, a torque control section is comprised of the current command value calculating section 101 and the maximum output limiting section 102.
The subtracting section 103 calculates a deviation current Iref3(=Iref2−Im) between the current command value Iref2 and a motor current Im of the motor 20 that is fed back, the deviation current Iref3 is controlled by a current control section 104 such as a PI control (proportional and integral control) or the like. Then; the controlled voltage control value E is inputted into a PWM control section 105 and the duty command values are calculated in synchronous with a saw-tooth carrier signal CS, having a predetermined frequency, generated in a carrier signal generating section 107, and in accordance with PWM-signals PS that the duty command values are calculated, the motor 20 is driven through an inverter 106. The motor current Im of the motor 20 is detected by a current detecting circuit 120 within the inverter 106, and the detected motor current Im is inputted into the subtracting section 103 to feed back. In a case that a brushless DC motor as the motor 20 is used for a vector-control, a resolver 21 as a rotation sensor is connected to the motor 20, and an angular speed calculating section 22 for calculating an angular speed ω from a motor angle (rotation angle) θ is provided.
A bridge circuit that bridge-connects semiconductor switching elements (e.g. FETs) and the motor 20 is used in the inverter 106 that controls the motor current Im by means of the voltage control value E and drives the motor 20, and the motor current Im is controlled by performing ON/OFF controls of the semiconductor switching elements in accordance with the duty command values of the PWM-signal determined based on the voltage control value E.
In the case that the motor 20 is a three-phase (U-phase, V-phase and W-phase) brushless DC motor, details of the PWM control section 105 and the inverter 106 is a configuration such as shown in FIG. 3. That is, the PWM control section 105 comprises a duty calculating section 105A that inputs each-phase carrier signal CS and calculates PWM-duty command values D1˜D6 of three phases (U-phase, V-phase and W-phase) in accordance with a predetermined expression based on the voltage control value E, and a gate driving section 105B that drives each gate of FET1˜FET6 by the PWM-duty command values D1˜D6 to turn ON/OFF. The inverter 106 comprises a three-phase bridge having upper/lower arms comprised of a U-phase upper-stage FET1 and a U-phase lower-stage FET4, upper/lower arms comprised of a V-phase upper-stage FET2 and a V-phase lower-stage FET5, and upper/lower arms comprised of a W-phase upper-stage FET3 and a W-phase lower-stage FET6, and drives the motor 20 by being turned ON/OFF with the PWM-duty command values D1˜D6. Further, electric power is supplied to the inverter 106 from the battery 13 through a power-source relay 14.
In such a configuration, although it is necessary to measure a drive current of the inverter 106 or the motor current of the motor 20, as one of request items of downsizing, weight saving and cost-cutting of the control unit 100, a singularity of the current detecting circuit 120 is proposed. A 1-shunt type current detecting circuit is known as the singularity of a current detecting circuit, and for example, the configuration of the 1-shunt type current detection circuit 120 is shown in FIG. 4 (for example, Japanese Published Unexamined Patent Application No. 2009-131064 A). Namely, a shunt resistor R1 is connected between the lower-stage arm of the FET bridge and the ground (GND), a fall voltage that is caused by the shunt resistor R1 when a current flowed in the FET bridge is converted into a current value Ima by an operational amplifier (a differential amplification circuit) 121 and resistors R2˜R4, and further the current value Ima is A/D-converted at a predetermined timing by an A/D converting section 122 through a filter comprised of a resistor R6 and a capacitor C1, and then a current value Im that is a digital value is outputted. Moreover, a voltage “2.5V” being a reference voltage is connected to a positive input terminal of the operational amplifier 121 through a resistor R5.
In a case that the currents for respective UVW-phases are detected by the 1-shunt type current detecting circuit, for example as disclosed in Japanese Published Unexamined Patent Application No. 2010-279141 A (Patent Document 1), a method that a judgement of the maximum duty, the intermediate duty and the minimum duty is performed and then the judged duties are sequentially arranged with respect to the shifted carrier period, is used. That is, the duty setting values for respective phases are compared, an then the maximum duty, the intermediate duty and the minimum duty are determined, as a reference being a rising phase Y of the carrier signal of the intermediate phase, a rising phase of the carrier signal of the maximum phase is led by a constant amount as well as a rising phase of the carrier signal of the minimum phase is lagged by a constant amount. Whereby the PWM-signals for the respective phases are generated based on the respective-phase carrier signals of which phases are sifted each other and the respective-phase duty setting values, and the current detection is performed in predetermined sections (periods) Tu and Tw till the respective risings of the PWM-signal of the intermediate phase and the PWM-signal of the minimum phase so as to be possible to detect the respective-phase motor currents by the single current detecting circuit.